Delayed automatic volume control circuit



Oct. 29, 1935. c. TRAVIS 2,018,982

DELAYED AUTOMATIC VOLUME CONTROL CIRCUIT Filed Nov. 5, 1932 2 Sheets-Sheet 1 INVENTOR CHARLES T V15 BY Mm v TTORNEY 4 Oct. 29, 1935. c. TRAVIS 2,018,932

DELAYED AUTOMATICVOLUME CONTROL CIRCUIT Filed Nov. 5, 1932 2 Sheets-Sheet 2 AAAAAA INVENTOR CHARLES TRAVIS BY Wi m ATI'ORNEY CPL Patented Oct. 29, 1935 PATENT OFFICE DELAYED AUTOMATIC VOLUME CONTROL CIRCUIT Charles Travis, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application November 5, 1932, Serial No. 641,350

Claims.

My present invention relates to automatic volume control arrangements for radio receivers, and more particularly to improved methods of, and means for, controlling the gain of a high frequency amplifier to maintain a substantially uniform amplified carrier intensity level.

The use of a diode detector, or rectifier, in a radio receiver to furnish automatic volume control bias to a preceding high frequency amplifier is attended with certain distinct disadvantages. In the first place, comparatively large radio frequency voltages must be applied to the diode oontrol element to give the necessary bias voltage to produce the automatic volume control function. In the second place, the generated bias is directly proportional to the amplified carrier; hence, the automatic volume control characteristic is not appreciably fiat unless a large number of tubes are controlled. Hence, it may be stated that it is one of the main objects of the present invention to overcome these last named disadvantages, and to furnish large automatic Volume control biases with comparatively small radio frequency inputs.

Again, in the usual and well known automatic volume control arrangement for radio receivers, voltage delay, devices are utilized for regulating the automatic volume control function in a manner such that the function does not become effective until a certain carrier input level is attained. However, in these usual voltage delay devices it has been found that if the carrier is modulated, the modulation peaks flash-over the fixed bias which results in an undesirable type of distortion. Therefore, it may be stated that it is another important object of the present invention to provide a voltage delay arrangement for an automatic volume control system wherein the automatic volume control bias will be independent of modulation, and the audio frequency output will be free from distortion due to the use of such a delay arrangement.

The aforementioned desirable functions of the present invention require the use of a rectifier for producing from the modulated carrier energy a control voltage which comprises direct current and alternating current components, a direct current amplifier which amplifies both components, and an additional device of asymmetric conductivity which functions to produce the control and delay action. The method employed in the present invention to coordinate these aforementioned elements is not only carried out in a novel manner, but the elements themselves are economically embodied within a single electron discharge device.

Accordingly, it may be stated that it is another object of the present invention to provide an automatic volume control system for a radio receiver wherein a single electron discharge tube has circuits associated with it in such a manner that the tube is capable of performing three independent functions which cooperate to furnish large volume control biases with comparatively small radio frequency inputs.

Still other objects of the present invention are to improve generally the simplicity and. efiiciency of automatic volume control arrangements for radio receivers, and to particularly provide an arrangement of this kind which is not only durable and reliable in operation, but economically manufactured and assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 diagrammatically shows a superheterodyne type of radio receiver embodying one form of the invention,

Fig. 2 shows a modification of the invention,

Fig. 3 shows a further modification of the invention.

Referring now to the accompanying drawings wherein similar reference characters designate the same elements in the different figures, in Fig. 1 there is shown merely by way of illustration a superheterodyne type of radio receiver, those portions of the receiver not essential to an understanding of the present invention being merely designated in conventional manner. Thus, the grounded antenna circuit A is shown coupled, as at M1, to the resonant input circuit of a stage of tuned radio frequency amplification including a tube I which may be of. the screen grid type, of the variable mu type, or even of the radio frequency pentode type. The anode circuit of tube is coupled, as at M2, to the resonant input circuit of the first detector, and the first detector has impressed upon it the output of the conventional local oscillator, the latter including a tunable circuit.

The resonant circuits of the amplifier I, the

2 first detector and the local oscillator each including variable tuning condensers 2 which are arranged for mechanical coupling by a uni-control device 3, and, as is well understood by those skilled in the art, the tuning range is such that a constant intermediate frequency is delivered to the output circuit 4 of the first detector throughout the tuning range of the receiver. The circuit 4 is maintained fixedly tuned to the desired intermediate frequency, which may have a value of between 175 to 465 kilocycles, and the circuit 4 is coupled, as at M3, to the fixedly resonant input circuit of the intermediate frequency amplifier tube 6, this tube also being of the same type as the tube l.

The output circuit '5, as well as the circuit 5,

.is resonant to the intermediate frequency, and

is coupled, as at M4, to the resonant input circuit 8 of the second detector stage which includes the electron discharge device 9, the circuit 8 being resonant to the desired intermediate frequency.

In the low potential sides of the grid circuits of tubes l and 6 are disposed blocking condensers is, and leads H, II respectively connect the grid circuits of tubes 1 and 6 to a common lead l2, the latter lead being connected to the automatic volume control arrangement to be later described. The cathode leads l3 and l3 respectively of tubes l and 6 are connected to a common lead i l which is connected to a portion of the automatic volume control arrangement which is also to be described at a later point. The tube 9, and its associated circuits, performs a triple function in the present automatic volume control arrangement. The tube is of the type known as an RCA- tube, and also known as a duplexdiode triode.

It is believed sufficient for the purposes of a clear understanding of the present invention to describe the tube as consisting of an envelope l5 within which is disposed a cathode I6, a control grid ll, an anode l8, an auxiliary cold electrode I9,and an additional auxiliary cold electrode 20. It will be noted that the cold electrodes l9 and 20 are disposed on either side of the cathode, the cathode being of the heater type, outside of the electron stream flowing through the mesh of the control grid H to the anode iii. In other words, the cathode has one emission surface for the triode including grid ll and anode l8, and another emission surface for the two rectifiers comprising the cold electrode I9 and the cathode emission surface adjacent it, and the cold electrode 20 and the electron emission surface adjacent it.

In actual construction, it may be pointed out that the two cold electrodes l9 and 2b are placed around a cathode, the sleeve of which is common to the triode unit. The cold electrode IE], or auxiliary anode, is connected to the high potential side of the resonant input circuit 8, while the cathode lead of the tube is connected to ground through a resistor R4, the point K between the cathode and the resistor R4 being connected to the low potential side of the input circuit 8 through a resistor R1 shunted by a condenser C1. The anode I8 is connected by a lead 2| to the positive end of the voltage supply divider P, the lead It from the cathodes of the controlled tubes l and 6 being connected to point 22 on the divider P. The lead l2 from the grid circuits of tubes l and 6 is connected to a desired point Y less positive than the point 22 through a path which includes the resistor R3, and the resistor R2, the latter resistor being arranged for adjustable tapping to secure the point Y so as to provide a manual sensitivity control.

The auxiliary cold electrode 28 is connected, by a lead 23, to a point intermediate the resistors R2 and R3. The point K on the grounded cathode lead of tube 9 is connected to the audio frequency amplifier, which amplifier may include one or more stages followed by a reproducer, through a path which includes the lead 24, the

direct current blocking condenser 25, and the grounded coupling resistor 26, if resistance coupling is employed between the second detector and the succeeding audio frequency amplifier.

A condenser C3 is connected between the cold electrode 20 and the lead M for a purpose to be presently described. The control grid ll of tube 9 is connected to the negativeend of the resistor R1 through a resistor R5, while the control grid is additionally connected to an intermediate point of the resistor R1 through a condenser 30 and an adjustable tap X, the latter providing a manual volume control device.

Appropriate by-passing condensers are utilized wherever necessary, and these condensers may generally be referred to by the reference numeral 40. It will be noted that they are connected to ground in each case and to the leads 2|, l4, l2 and are also utilized in connection with the resistor R2.

The operation of the present invention, as shown in Fig. 1, can now be explained. The cold electrode is and its associated cathode emission surface, as well as the input circuit 8, constitute a detector with leak resistor R1 and the by-pass condenser C1. turns to the cathode at point K, and not to ground. The detected direct current and audio frequency components are impressed together on the control grid I! of the triode amplifier portion of the tube 8; this triode amplifier has its output coupling resistance R4 in the cathode leg and has its plate l8 efliciently by-passed to ground for both direct current and audio frequency by the power system and filter. Hence, amplified voltages, both direct current and alternating current, will appear across the resistor R4.

The point Y on the divider P is held positive with respect to ground, and the cathodes oi. the tubes to be controlled are connected to a point 22 more positive than the point Y by an amount equal to the desired initial bias, and this desired initial bias can be adjusted by means of the variable tap to point Y thereby furnishing a manual sensitivity control. The resistors R2 and R3 are large, of the order of 0.5 or 1.0 megohm. The condensers are audio frequency filter condensers, while the condenser 25 is the usual audio frequency coupling condenser into the next audio amplification stage.

It has been pointed out heretofore that the anode 20 is connected to the cathode point K through a condenser C3, and it is additionally to be noted that this condenser should be large enough to short circuit the path between anode 20 and its adjacent cathode surface for audio frequency. For weak signals the point K is more positive than the point Y, and hence more positive than the cold electrode 20. Therefore, no space current fiows to the electrode 20, but on account of the heavy condenser C3, the electrode 20 follows point K for the audio cycle. Under these conditions the automatic volume control bias is that determined by the potential of point Y; that is to say, it is constant and equal to the initial bias,

The input circuit 8 rethe audio frequency being filtered oif by the resistor R3 and the by-pass condenser 49.

For sufiiciently strong signals, the point K becomes more negative than the point Y (for direct current), and the plate l9 draws space current, thus eifectively shorting the junction of resistors R2 and R3 to point K. The automatic volume control bias is thus determined by the direct current potential of point K, and increases with it as the signal increases. But, as the plate 29 stillfollows-the point K for audio frequencies, any additional audio frequency load is constant over the audio cycle and does not produce distortion. Therefore, the automatic volume control bias depends only on the direct current potential of the point K, and hence, only upon the carrier input andnot .upon the modulation. With the 55 tube 9 a voltage amplification of about 6 can be attained. Accordingly, 10 volts peak carrier will give an available bias of 60 volts, of which, for example, 40 may beheld back by the delay action, and the last 20 used to control the radio frequency tubes l and 6.

Normally, the potential of the point Y is made less positive than the potential of the point K so that no current will flow through the path including the plate 26 and the resistor R2. As signals increase, the tube resistance increases thus making the point Y more positive with respect to the point K. This means that at critical input, the current suddenly starts through the resistance R2 .thus making the controlled grids'negative. The controlled cathodes are conneci'ed to point 22 which is slightly more positive than the point Y to provide the normal bias for weak signals.

It will now be seen that the tube 8 and its associated circuits performs three independent, but cooperating, functions. The anode l9 and its adjacent electron emission surface functions as a detector, the grid ll, anode l8 and associated electron emission surface functions as an amplifier for the detected output of the last named detector, while the anode 28 and its associated emission surface functions as a valve for preventing any variaion of the automatic volume control bias until the input has reached the value which lowers the potential of the cathode to a predetermined level.

It is not essential to take off the audio voltages from the cathode of tube 9 as shown. They may be taken off in the usual way from the plate side of the circuit, the cathode resistor being then by-passed for audio frequency, and used as a direc': current impedance only. A modified circuit that will accomplish this is shown in Fig. 2.

In the circuit of Fig. 2 the elements similar to the element shown in connection with tube 9 and its associated circuits, are similarly desig nated. The operation is much the same as before, except that very little of the audio voltage appears at point K owing to the high impedance of the primary of the audio frequency coupling transformer 51, and the by-pass effect of the condenser 49. The heavy condenser 03 is retained, however. to prevent any distortion at the point where the cold electrode 2i! is beginning to draw space current, for very low audio frequencies.

If push-pull output without transformer coupling is desired the modification of Fig. 3 may be used. A pair of 47 tubes can be driven to full grid swing in this manner with somewhat more than 20 volts peak carrier, 20% modulated, at

the detector diode, an input which'is considerably less than that employed in many existing receivers to provide sufficient automatic volume control directly from the diode. In this circuit'half of the direct current amplification is necessarily 5 thrown away, but this is less important in View of the larger signal input. Thus, in the example iven 89 volts of automatic volumecontrol bias is available, all or most of which is used as a delay. It is not believed necessary to describe fur- 10 ther the construction of Fig. 3 since theelements shown correspond to those already described in connection with Figs. 1 and 2.

It has already been explained that the tap at point Y provides a. manual means of controlling the maximum sensitivity of the receiver. The delay voltage, ,or the potential of the cathode above point Y for most signal input depends only to a small extent upon the value of the cathode resistor, but is determined largely by the location 2 of the plate connection to the B voltage divider and the resulting plate voltage. The control by means of the tap X prevents overloading the plate circuit of the triode amplifier including grid El and anode it at high modulation.

The con rol at the point Y seems preferable to sensitivity control in the antenna in case so-called quiet automatic volume control, or noise suppression. is employed. As far as automatic volume control action is concerned, the direct current 3,0v amplification of the controlling bias is equivalent to a corresponding increase in the radio frequency gain of the receiver, and this increase may be used if desired to make the automatic volume control characteristic flatter, or to increase threshold sensitivi y, or both. The increase in gain possible is very nearly 8 for the 55 tube,'so that the threshold may be moved down to an antenna input of one-eighth of what it was without the use of direct current amplification, the 40 delay voltage remaining the same; or, on the other hand, the delay may be multiplied by 8 for the same threshold.

The indirect advantages accruing from the use of direct current amplification are probably quite as imporant as the more apparent advantage that has just been discussed. It is regarded as conservative to say that the delay voltage should be at least as great as the bias necessary to control the strongest signal to be encountered. This gives a 2:1 rise in output from threshold to maximum input, and in the average receiver necessitates a delay of 39 to it) volts. The diode driver (last intermediate frequency tube) is therefore called upon for an ou! put of volts peak car- 55 rier or more at the diode plate for the strongest signals, and at this output must drive two diodes, one for detection and one for automatic volume control. At the threshold the automatic volume control diode places no additional load upon the 50 driver, but as the signal strength increases the load from this source increases correspondingly.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to oneskilled in the art that my 5 invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. An amplifier to be controlled as to the gain thereof, a rectifier, a second amplifier, means for impressing the direct current component of the rectifier output upon an input electrode of the" second amplifier, adjustable capacity means for impressing a predetermined. portion of the alternating current component of the rectifier output upon the second amplifier input, electron discharge'device of asymmetric conductivity, means common to the space current'paths of the second amplifier and device for impressing the second amplifier output on said device whereby the output of the second amplifier produces negligible eiiect in the output of said device until a desired threshold value is attained, said rectifier and device being disposed within a common evacuated envelope, a direct current connection between the output of said device and said controlled amplifier, the gain of said controlled amplifier being controlled from said device only after said value is reached.

2. An amplifier to be controlled as to the gain thereof, a rectifier, a second amplifier, capacity means, adjustable with respect to relative amounts of alternating current and direct current components of the rectifier that can be imposed on the second amplifier, for impressing the said alternating current component of the rectifier output upon the second amplifier input, means for impressing said direct current component upon an input electrode of the second amplifier, a device of asymmetric conductivity, means for impressing the second amplifier output on said device whereby the output of the second amplifier produces negligible effect in the output of said device until a desired threshold value is attained, a direct current connection between the output of said device and said controlled amplifier, the gain of said controlled amplifier being controlled from said device only after said value is reached.

3. An amplifier to be controlled as to the gain thereof, a rectifier, a second amplifier, capacity means, adjustable with respect to relative amounts of alternating current and direct cur rent components of the rectifier that can be imposed on the second amplifier, for impressing the said alternating current component of the rectifier output upon the second amplifier input, means for impressing said direct current component upon an input electrode of the second amplifier, a device of asymmetric conductivity,

means for impressing the second amplifier output on said device whereby the output of the second amplifier produces negligible effect in the output of said device until a desired threshold value is attained, a direct current connection between the output of said device and said controlled amplifier, the gain of said controlled amplifier being controlled from said device only after said value is reached, an audio frequency amplifier, and means including a portion of the external cathode-plate path of the second amplifier for impressing the audio frequency component of the second amplifier output on the audio frequency amplifier. V

4. In a radio receiver provided with a high fre quency amplifier including a grid and cathode, a delayed automatic volume control arrangement comprising a tube having a cathode, control grid, anode, and a pair of diode anodes disposed adjacent the cathode outside the electron stream between the cathode, grid and first anode, a high frequency network connecting one of the diode anodes to the cathode, said network being coupled to the amplifier, an impedance in said network for developing a direct current component and an audio component from signal energy impressed on the network, a voltage divider, a connection from the first anode to a positive point of said divider, a connection from the amplifier cathode to a less positive point on the divider, an impedance connected between the other diode anode and a point on the divider less positive than said second point, an impedance connected between said second cathode and a point on said divider less positive than said third point, a direct current connection between the negative side of said first impedance and said control grid, a condenser adjustably connected between aid control grid and said first impedance, and a gain control connection between said second impedance and the grid of said amplifier.

5. In a radio receiver provided with a high frequency amplifier having a grid and cathode, a delayed automatic volume control arrangement comprising a tube having a cathode, control grid, anode, and a pair of diode anodes disposed adjacent the cathode outside the electron stream between the cathode, grid and first anode, a high frequency network connecting one of the diode anodes to the cathode, said network being coupled to the amplifier, an impedance in said. network for developing a direct current component and an audio component from signal energy impressed on the network, a voltage divider, a connection from the first anode to a positive point of said divider, a connection from the amplifier cathode to a less positive point on the divider, an impedance connected between the other diode anode and a point on the divider less positive than said second point, an impedance connected between said second cathode and a point on said divider less positive than said third point, a direct current connection between the negative side of said first impedance and said control grid, a gain control connection between said second impedance and the grid of said amplifier, an audio frequency amplifier, and means including said second impedance for impressing the audio component in the first anode circuit upon said audio amplifier.

' CHARLES TRAVIS. 

